Pneumatic rubber tires are conventionally composed of a toroidal carcass with a circumferential tread and adjoining sidewalls.
The tread is conventionally designed to be ground-contacting and thereby is of a suitable rubber composition for such purpose and also is of a suitable configuration. For example, such a tread may have properties which emphasize good traction and resistance to treadwear. Such tires may have a tread of a lug and grove configuration which is designed to be ground-contacting.
In contrast, the associated sidewalls are conventionally not designed to be ground contacting and, in order to be appropriately supportive of the tread, are conventionally composed of a rubber composition which is not designed for traction and resistance to treadwear but, instead, of a rubber composition which is of low hysteresis to provide low heat buildup and is softer to provide greater flexibility than that of the tread rubber.
As a result, such softer sidewall rubber compositions typically have less resistance to puncturing objects, abrasion and to scuffing against road curbs or other objects.
However, some tires are desired to be utilized under more harsh conditions such as, for example, rough roads or off-the-road service or on specialty vehicles which may be designed to run on low inflation tires where the tire sidewall may come in contact with the ground. Such circumstances may occur, for example, in various mine operations and in logging operations, or where the tire is run over rough terrain at low inflation pressures. Under such conditions, growth of a crack, or cut, in the sidewall and, also, resistance to puncture, are significant considerations.
A significant need for a sidewall rubber composition for such use is a hybrid rubber composition which with suitable resistance to puncture and abrasion typically associated with tread rubber compositions while substantially maintaining flex fatigue and hysteretic properties typically associated with sidewall rubber compositions.
It is appreciated that commercially available tires may sometimes have sidewalls designed to be resistant to cut growth which are comprised of carbon black reinforced elastomer compositions comprised of natural rubber and cis 1,4-polybutadiene rubber.
It is also appreciated that carbon blacks for reinforcement of tire sidewall rubber compositions are conventionally of appreciably larger particle size than carbon blacks usually used for tire treads since resistance to heat buildup rather than abrasion resistance is typically more important for tire sidewalls.
Such carbon black might be exemplified, for example, by having an Iodine value (number) (ASTM D1510) in a range of about 35 to about 85 g/kg instead of a higher iodine value of at least about 105 which would be more representative of a carbon black typically used for a tread rubber composition.
Here, however, it is desired to provide a novel tire with rubber sidewall of a lug and groove configuration an of a composition comprised of natural rubber and cis 1,4-polybutadiene of which a significant portion is designed to be occasionally ground contacting and thereby having a resistance to puncture as well as resistance to abrasion more typical of tire tread rubber compositions yet substantially retaining flexibility and low hysteresis usually required for a tire sidewall.
It may be readily thought of to increase the natural rubber content of the sidewall composition in order to increase resistance to tear. However the cis 1,4-polybutadiene content would be correspondingly reduced which would be expected to thereby undesirably reduce its resistance to abrasion and undesirably increase its hysteresis (as would be expected to be evidenced by a reduction in its rebound property).
Accordingly, it is desired herein to provide such a tire sidewall of a rubber composition which contains large particle size carbon black reinforcement common for sidewall compositions and which is composed of natural rubber and cis 1,4-polybutadiene elastomers, also somewhat common to many sidewall compositions but, however, which has enhanced resistance to abrasion and puncturing objects and, also enhanced resistance to tear more common to tire tread rubber compositions.
For the purposes of this invention, rubber reinforcement for said lug and groove configured sidewall is composed of a combination of said carbon black as well as a starch/plasticizer composite, aggregates of precipitated silica and coupling agent.
In the description of this invention, the term “phr” relates to parts by weight of an ingredient per 100 parts by weight of rubber.
The terms “rubber” and “elastomer are used interchangeably.
The term “Tg” relates to a glass transition temperature of an elastomer, normally determined by a differential scanning (DSC) calorimeter with a temperature rise of 10° C. per minute. (ASTM D3418)